abstract override inconsistency?

Using a an example from Martin's book, I'm (once again) wondering if this behavior is buggy, or some spec corner case I'm unaware of.

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scala> abstract class IntQueue {
     |   def get(): Int
     |   def put(x: Int)
     | }
defined class IntQueue

scala> trait Doubling extends IntQueue {
     |   abstract override def put(x: Int) { super.put(2 * x) }
     | }
defined trait Doubling

scala> class BasicIntQueue extends IntQueue with Doubling {
     |   private val buf = new scala.collection.mutable.ArrayBuffer[Int]
     |   def get() = buf.remove(0)
     |   def put(x: Int) { buf += x }
     | }
<console>:10: error: overriding method put in trait Doubling of type (x: Int)Unit;
 method put needs `override' modifier
         def put(x: Int) { buf += x }
             ^

Yet this works:

scala> class BasicIntQueue extends IntQueue {
     |   private val buf = new scala.collection.mutable.ArrayBuffer[Int]
     |   def get() = buf.remove(0)
     |   def put(x: Int) { buf += x }
     | }
defined class BasicIntQueue

scala> val dq = new BasicIntQueue with Doubling
dq: BasicIntQueue with Doubling = $anon$1@647bb210

scala> class DoublingIntQueue extends BasicIntQueue with Doubling
defined class DoublingIntQueue

I fail to see the structural differences, particularly linearization seems identical, no?